1. Field of the Invention
The present invention relates to an apparatus and method for evaluating magnetic heads. More particularly, the invention relates to a technique of evaluating GMR heads before they are incorporated into magnetic disk drives. The invention relates to an apparatus, a method and a disk that can evaluate GMR heads that cause no magnetic inversion at the GMG-head supporting layer of a magnetic disk drive after a GMR head is incorporated into the magnetic disk drive.
2. Description of the Related Art
In recent years, magnetic disk drives have become smaller and have acquired a larger storage capacity. It is therefore increasingly demanded that magnetic disk drives should magnetically reproduce data from disks at higher sensitivity. Developed recently as a magnetic head with use of a magnetoresistive effect is the GMR head (also known as spin-valve head).
The head element of the GMR head has a spin-valve film that is only 20 nm thick. The head element is inevitably very weak to ESD as a result of static electricity. Consequently, the GMR head has a withstand voltage of only about 5 V, which is far smaller than that (about 100 V) of the inductive thin-film heads that were widely used before the GMR came into general use.
The GMR head has a breakdown mode, as the studies made hitherto reveal. The breakdown is a magnetic breakdown that results from the large-current magnetic field caused by the above-mentioned ESD. The magnetic breakdown is a phenomenon in which the magnetic field generated from a large current impairs the magnetic stability in the spin-valve film, inevitably forming a magnetic domain, although the spin-valve film is not physically broken.
More specifically, the GMR element has a fixed layer (PIN layer), a free layer, and an anti-ferromagnetic layer. The GMR element generates a positive output when its resistance increases as the fixed layer and the free layer are magnetized in opposite directions. It generates a negative output when its resistance decreases as the fixed layer and the free layer are magnetized in the same direction. The fixed layer is oriented, opposing the signal magnetic flux emanating from a recording medium that lies at right angles to the track-width direction, because the anti-ferromagnetic layer achieves exchange coupling. By contrast, the free layer is oriented parallel to the signal magnetic flux emanating from a recording medium or parallel to the track-width direction, because permanent magnets are provided at the ends of the free layer to control the magnetic domain. Hence, the magnetization direction of the fixed layer is important, determining the polarity (positive or negative) of the output waveform.
When an ESD flows in the fixed layer, a magnetic field of the opposite direction is generated, inverting the magnetization direction (PIN direction) of the fixed layer. Namely, magnetic breakdown (PIN inversion) takes place. The magnetic breakdown results in neither resistance change nor shape change, but causes an abnormal waveform in the output of the GMR element. For example, the magnetic breakdown makes the reproduced waveform unstable or changes the amplitudes of the positive and negative halves of the reproduced waveform If this magnetic breakdown occurs, the magnetic head cannot correctly detect the data. This causes errors in the data or makes it impossible to reproduce the servo signals. Then, the magnetic head can no longer be accurately positioned. Hitherto, any head that is found in the head test (HT) to cause such magnetic breakdown is discarded and not used in magnetic disk drives.
Further, a method of evaluating magnetic heads is known (see, for example, Jpn. Pat. Appln. Laid-Open Publication No. 2001-6133). In the method, a magnetic head records data patterns on a recording medium, one pattern for time T1 between a positive pulse and a negative pulse, and another pattern for time T2, between a negative pulse and a positive pulse, shorter than time T1. Then, the data patterns are reproduced from the recording medium. Time T2 measured of a data waveform reproduced is subtracted from time 1 measured of the data waveform. If the difference, (T1−T2), has a negative value, the direction of magnetization of the fixed layer of the GMR element is considered to have inverted.
However, a magnetic head that is found to be a good one in the test may reproduce a data waveform inverted in polarity with respect to a normal data waveform in the final test carried out after the magnetic head has been incorporated into a magnetic disk drive. Consequently, the data signals or the servo signals that the head has reproduced are erroneous. This is probably because the magnetic head has ESD not so prominent as to cause waveform inversion and therefore passes the first test, and is incorporated into a magnetic disk drive, but receives a small impact as a result of, for example, contamination, inevitably generating a inverse magnetic field and hence inverting the direction of magnetization of the GMR element. (The small impact is perhaps micro-ESD, which does not usually result in polarity inversion in the fixed layer of the GMR element.)